1. Field of the Invention
The present invention relates to heat exchanger. The present invention particularly relates to a heat exchanger for use in devolatilizing polymers.
2. Background of the Art
Polymers are a major article of commerce in a wide variety of industries. They are used in the manufacture of houses and automobiles. Polymers are also widely used in the practice of medicine where they are incorporated into articles such as syringes, plasma bags, specimen holders, and protective garments. Polymers are even used in the distribution of foodstuffs wherein the polymers, for example, can serve to protect food from contamination and spoilage.
Many polymers are produced using solvents, low molecular weight monomers, or both. For example, polystyrene can be prepared using solution or bulk polymerization. In the process of preparing polystyrene, the polymerization reaction is typically discontinued when polymerization of 40 to 90 percent by weight of monomer is obtained. The unreacted monomer and volatile substances remaining in the resultant polymer, and solvent also remaining in the polymer in the case of the solution polymerization, are desirably separated. It is especially desirable to remove residual volatile components in the case of styrene polymer used for food containers or the like.
When performing a devolatilization operation, it can often be important to avoid exposing the polymer being treated to excessive temperatures and even to moderate temperatures for too long a period of time. The possible results of failing to do so can be excessive polymer molecular weight, undesirable crosslinking, or even thermal polymer degradation.
In order to remove the volatiles from polymers and polymer solutions, an apparatus for removing volatile components is employed. For example, EP 0 359 432 B1 to Morita, et al., discloses the use of a heat exchanger for giving a polymer solution an amount of heat necessary for the evaporation of the volatile components and also heating the polymer solution after the evaporation of the volatile components to maintain adequate fluidity of the polymer, a devolatilizer for causing evaporation of volatile components contained in the polymer solution a vacuum source, and a distributor located between the heat exchanger and devolatilizer.
In the case of highly viscous polymers, another approach is disclosed in U.S. Pat. No. 4,808,262 to Aneja, et al. Therein it is disclosed to use a plate heat exchanger to wherein the polymer is heated along a short zone of indirect heat exchange. It is further disclosed that the residence time within the zone of indirect heat exchange ranges from approximately 5 seconds to 120 seconds.
The use of plate heat exchangers is also disclosed in U.S. Pat. No. 5,453,158 to Cummings, et al. Therein, it is disclosed to use a polymer devolatilization apparatus including a flat plate heater having a multiplicity of flat plates defining a plurality of channels, each channel having a substantially uniform height but varying width over its length, each channel including three zones. The a first zone is described as being in operative communication with the polymer solution supply device, characterized by decreasing width as a function of distance from its beginning. The second zone is described as beginning at the terminus of the first zone, and characterized by at least one occurrence of a restrictive cross-sectional area. The third zone is described as beginning at the end of the second zone and terminating at a liquid/vapor collection and separation region operating at reduced pressure, and being characterized by increasing width as a function of distance from its beginning. It is also disclosed that the ratio of maximum width of the third zone to the maximum width of the second zone is from 2:1 to 20:1.